The presence of unwanted impurities in a semiconductor material can have drastic results on the conductive properties of the semiconductor, even where such impurities are present at extremely low concentration levels. Because the unwanted impurities must in many instances be no greater than a few parts per billion, the techniques employed to purify the starting materials must be extremely rigorous to achieve the exacting standards imposed.
Various techniques have been developed wherein an impure starting material is treated to remove very high percentages of undesirable impurities mixed therewith. For example, starting materials can be subjected to cryogenic distillation or pressure distillation, or gaseous diffusion, or solid phase zone refining. However, in those techniques, as well as with other available purification methods, the level of purity which must be consistently maintained to produce semiconductor-grade material in large quantity either cannot be reached at all or can be attained only at a prohibitive cost.
After the starting material has been purified to semiconductor-grade standards, it still must be further processed to produce a conductive semiconductor film having acceptable physical and electronic properties. Where the semiconductor is produced from a single material such as silicon, combined with a dopant, the manufacturing process is less complex than that wherein two or more materials are combined in conjunction with dopant to produce a binary, ternary or higher semiconductor film.
Semiconductors are produced by the successive build-up of individual layers of semiconductor-grade material and dopant. To derive reproducible electronic characteristics from a silicon film, it is necessary for the silicon film manufacturer to introduce a precise amount of uniformly dispersed electronically-active dopant. The resulting film should appear under high magnification as a uniform plane of silicon with interspersed dopant material. The presence of impurities in the film interrupts the planar uniformity and results in stress points which can alter both the structural and electronic integrity of the film. It can be appreciated that a film comprised of two, three, or more components of different atomic geometries in addition to a dopant will be more susceptible to structural and electronic defects than a silicon film unless the components, essentially free of adulterants, are uniformly combined in precise ratios. It is also highly desired that the deposited layer of semiconductor-grade material and dopant be unaffected by the deposition of additional layers. Where deposition occurs by thermal decomposition, the dopant in the underlying layers may diffuse from one layer to another or may migrate within a single layer if the temperature of deposition is too high. High temperatures are necessary to deposit semiconductor materials which are resistant to decomposition, i.e., stable. It is preferred that the semiconductor material be relatively unstable, to facilitate decomposition; however, such materials are difficult to purify because of their instability. Ideally, such relatively unstable semiconductor materials would be purifiable under moderate conditions which remove undesired components and permit thermally-induced deposition at temperatures which do not result in dopant diffusion or migration.
Silicon has long been used in the manufacture of semiconductors. However, other materials may be employed to manufacture semiconductors, and in certain instances these materials possess electronic characteristics superior to those of silicon. A silicon atom has four valence electrons; in a lattice of silicon atoms, the overwhelming majority of valence electrons act as a "glue" to bond the crystal lattice together. Because very few of the silicon valence electrons are available to conduct current, the silicon lattice is a relatively poor conductor without the addition of another material (a dopant) which fits into the lattice and has either an excess or a shortage of electrons to facilitate passage of either negative or positive current. Combinations of elements such as gallium and arsenic or aluminum, gallium and arsenic may also be used to manufacture a crystal lattice to form a semiconductor. Gallium and aluminum are Group IIIa elements which have three valence electrons while arsenic is a Group Va element which has five valence electrons. A crystal lattice of gallium arsenide has the same number of available valence electrons as a silicon crystal. However, because of its different atomic properties, a gallium arsenide crystal causes the conduction electrons to move at a much higher velocity than that of silicon electrons. In gallium arsenide the maximum electron velocity is about 10.sup.8 centimeters per second, while in silicon the maximum velocity is about 2.times.10.sup.7 centimeters per second. Conduction electrons in an aluminum gallium arsenide crystal travel even faster than those in the gallium arsenide crystal. Additionally, electrons in the gallium arsenide crystal are less likely to collide with the lattice than is the case in a silicon crystal. The mean free path of an electron in moderately doped gallium arsenide is roughly ten times the mean free path of electrons in silicon. M. Heiblum, L. Eastman, "Ballistic Electrons in Semiconductors", Scientific American, 256, No. 2, p. 102 (1987).
Where the semiconductor film comprises two or more components, the need for the purification procedure to deliver a product having reproducible, low levels of impurities is more pronounced. Typically, the semiconductor film manufacturer purchasing already-purified starting materials will encounter substantial lot-to-lot variation in the actual level of impurities in the material as delivered. Such variation has marked effects on the conductivity of the resulting epitaxial film. In addition, the purification processes used by individual suppliers employing standard distillation and refining techniques have limitations as to the removal of certain impurities, such as materials which azeotrope with the desired starting material, compounds with very similar boiling points and organic compounds.
It is an object of this invention to provide a method whereby relatively impure, relatively unstable starting materials containing Group IIIa and Group Va elements are purified such that residual impurities consistently do not exceed parts per billion levels.
It is a further object of this invention to provide a process for the production of consistently high quality films by thermal decomposition from a variety of starting materials for use in semiconductor devices whereby dopant diffusion and migration are minimized.
Further objects and attendant advantages of the present invention will become better understood from the following description.